1. Technical Field
The present invention relates to nails. More particularly, the present invention relates to nails employing a combination of nail shank deformations to prevent axial withdrawal and shear slip.
2. Description of Related Art
Generally, in order to keep two or more objects fastened together, a nail must resist axial forces that act to separate the objects along the nail's longitudinal axis. In order to prevent axial separation, the friction between the surface of the nail shank and the inner surfaces of the bores in contact with the nail shank must be sufficient to resist the axial separating forces. With the nail as the frame of reference, an axial separating force acts on the nail in a direction toward the nail head, but friction acts in the opposite direction to keep the nail from moving relative to one or more of the fastened objects. When the friction is insufficient to resist an axial separating force acting on the nail, the forces can cause the nail to withdraw, or back out, from a bore in a fastened object.
A known problem with nails is their inability to resist withdrawal from the base materials into which they are driven. A common method to improve a nail's ability to resist withdrawal, also referred to as withdrawal performance, is to create mechanical deformations on the surface of the nail shank to increase the frictional resistance to axial forces along the shank and the bore surfaces.
One way to create mechanical deformations on the shank surface of a nail involves forming a series of annular rings axially spaced along the shank. In many applications, these ringed deformations provide good resistance to withdrawal. However, in addition to providing resistance to axial forces, a nail must also resist shear forces that act to move the objects relative to each other in a direction perpendicular to the nail's longitudinal axis. In this regard, the use of ringed deformations is often accompanied by a reduction in the nail's ability to resist movement of the fastened objects in a direction perpendicular to the nail's longitudinal axis, also known as shear slip. The annular rings generally have an outer diameter that is larger than the un-deformed sections of the shank. As a nail with annular rings on its shank is driven into a base material, the outer diameter of the annular rings creates an annular gap where the bore is larger than the diameter of the un-deformed sections of the shank. As a result, when a shear force acts on a fastened object where an annular gap exists, the nail can only resist the shear force when the object moves enough to allow the shank of the nail to make contact with the inner surface of the bore, which has a larger diameter.
Another way to create mechanical deformations on the shank surface involves forming a threaded, or screw-shaped, deformation. The helical pattern of a threaded nail shank creates corresponding helical channels in the object into which the nail is being driven. The nail rotates in one direction as it moves through the object, and the threaded deformation on the nail only travels within the helical channels in the object. Meanwhile, the other sections of the bore surface remain in contact with the rest of the shank. Thus, no annular gap is created, providing better shear slip performance than a nail with annular rings. Moreover, a nail with a threaded deformation encounters less resistance than a nail with annular rings when it is being driven into an object. However, a nail with a threaded deformation, though more effective than a nail without deformation, is still susceptible to withdrawal. A nail with a threaded deformation can slowly spin out of the base material, when forces, such as those resulting from vibration, cause the nail to rotate counter to its driving rotation, i.e., the direction it rotates when being driven into the base material.